Ionization chamber



Feb. 17, 1948.. B. L. WELLER IONIZATION CHAMBER Filed Nov. 27, 1943 M/ 89 t W Z d M .0

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WWI r ,M M U IJI JW lwb A24... a M z'iarnaw Patented Feb. 17, 1948IONIZATION CHAMBER Barton L. Weller, Chicago, 111., aeaignor to theUnited States of Ameri Atomic Energy Comm ca, as represented by thelesion Application November 27, 1943, Serial N6. 512,070

8 Claims. (Cl. 25l-83.6)

My invention relates to apparatus for measuring the intensity ofpenetrating radiations such as gamma rays. More specifically, it relatesto an ionization chamber having a collecting electrode and highpotential electrode and an outer envelope serving as a guard, togetherwith an asso-' ciated electrical metering circuit.

An object of my invention is to provide a device that is capable ofcreating measurable electrical currents in response to, and proportionalto, penetrating radiations that may impinge on it, and that is of suchsimple construction that it is readily adaptable for large scaleproduction.

Another object of my invention is to provide such a radiation measuringdevice that is safe to handle while in operation.

A further and more specific object of my invention is to provide an ionchamber having collecting and high potential electrodes in which theenvelope, together with associated guard rings internally and externallythereof, serves as a guard system for preventing the intenseelectrostatic fields ot the high potential electrode and leads fromdisturbing the measurement of the ionization currents.

These and further objects will become more apparent from the study ofthe following description taken together with the drawing, in which:

Fig. 1 is a longitudinal cross-sectional view of an ion chamber andassociated circuit showing the teachings of my invention; and Fig. 2 isa cross-sectional view taken on the line 2-2 of Fig. 1.

Referring more particularly to Fig. 1, numeral i denotes a metal chamberor envelope having welded thereto flanged header plates 2 and 3. Fusedto an eyelet portion 3a welded on plate 2 is an insulator 4 of glass, orany other similar material, which in turn is fused to a reentrantmetallic support for a rod or lead-in 5 connected to a cylindricalcollecting electrode 6, thus forming a glass to metal seal between theheader plate 2 and lead-in 5. The header plate 2 is also pro-' videdwith one or more flanged eyelet portions 1 having fused thereto glassbeads 8 which in turn are fused to rod 9 which serves as a lead-in andsupport by being spot welded to a high potential cylindrical electrodel0. High potential electrode I 0 may be either positive or negative withrespect to collecting electrode 6 depending upon whether it is desiredto have electrode 6 collect positive ions or negative ions,respectively. A metal tubulation H, which is capable of being sealedoil, is provided for filling and exhausting the envelope. The envelopeis filled with any desired gas such as argon or freon, preferably athigh pressures such as between 1 and 5 atmospheres. A high voltagesource such as a battery I! is connected between the high potentialelectrode lead-in II and envelope I. An electrical measuring instrument[3 such as an electrometer, galvanometer, or similar device, isconnected between the collecting electrode lead-in 5 and metalchamber 1. A flexible lead-in conductor l4 connects lead-in I to meter I3. A detachable connector II, of any suitable well-known design, may beused to make the conductor l4 readily detachable from lead-in I. A guardring l8 having a cover plate I! forms an electrostatic shield forshielding insulator 4 from the high potential leadin. This shielding iscontinued along conductor I 4 by the coaxial connector l5 and coaxialcable 22 which incorporate sheaths that completely surround conductor l4and are electrically connected to the header 2 and envelope I. An upperextension of chamber l, taken together with a cover plate I! and conduit23, forms an external electrostatic shield for completely shielding thehigh potential and collecting electrode lead-ins and preventing theirmechanical injury. Conduit 23 also prevents possible failure of theinsulation of lead-in I! from harming the operator. An internal guardring 21 is provided to protect collecting electrode insulator 4 from theelectrostatic eilects of the high potential electrode II and lead-in Q.

In operation, when the device is subjected to penetrating radiation suchas gamma rays, the high potential source applied to electrode III willcause ions formed in the chamber to move to-' wards the collectingelectrode and will cause current to flow through the meter l3. Sinceheader plate 2 is electrically grounded and interposed between bead 8and insulator 4, the high potential gradient created by battery i2appears only across head 8 and not across insulator 4, because only asmall potential exists across meter l3 and insulator 4. It will beobserved that leakage currents that tend to flow along the surface ofglass beads I, because of the high potential gradientv placed thereondue to the high voltage, will not enter the collecting electrode circuitbut instead will flow through the battery I! which is of sumcientcapacity to supply this negligible loss. Guard ring I electrostaticallyshields the collecting electrode from disturbing efiects caused by anycharge that may be present on the insulator" 4 moving over its surfaceas the result of the potential gradient from the strong electrostaticfield created by the high potential electrode lead-in the envelope aswell as internal and external guard rings, the collecting electrodemetering circuit is substantially isolated electrically andelectrostatically from disturbing variations due to the electrostaticeffects resulting from leakage from the surface of the high potentiallead-in insulator, thereby making it possible to measure very smallionization currents in the chamber even of the order of 10- an peres.Likewise, by

, virtue oi. the guard system, since a relatively low potential appearsbetween the header plate 2 and lead-in 5 (perhaps of the order of 10volts maximum), it is unnecessary to provide large and very highresistance insulating materials such as amber, for example-and instead,ordinary insulating materials can be used in the making of insulator 4.

It will be readily apparent that other modifications of my invention maybe made without departing from the spirit of the invention disclosed.

Iclaim:

1. Apparatus for measuring pentrating radiations comprising, incombination, a vacuum tight sealed metal envelope, a collectingelectrode and a high potential electrode contained therein, lead-insconnected thereto, insulators for in-- sulating said lead-ins from saidenvelope, a guard ring secured to said envelope and extending into thechamber formed thereby in a manner so as to electrostatically shield theinterior surface of said collecting electrode insulator from said highpotential electrode and lead-in, an electrical meter connected to saidcollecting electrode leadin, said collecting electrode being maintainedat substantially the same potential as said envelope, a voltage sourceconnected between said high voltage electrode and said envelopeexclusive of said collecting electrode lead-in, thereby draining leakagecurrents from the surface of said high voltage insulator to the voltagesource in preference to said collecting electrode or meter.

2. Apparatus recited in claim 1, together with metal guard meansexterior of said envelope and between said lead-ins forelectrostatically shielding ,the collecting electrode insulator from the.high potential lead-in.

3. An ionization chamber comprising, in combination, a metallic envelopehaving header plates sealed vacuum tight thereto by welds, a collectingelectrode and lead-in, an insulator forming. part of the envelope wallsand sealed to a, flanged portion of one of said header plates and tosaid lead-in, a high potential electrode andlead-in, a second insulatorsealed to another flanged portion of said header plate and to said highpotential lead-in, and an electrostatic shield exterior of said envelopeand between said collecting electrode insulator and high potentiallead-in.

4. Apparatus as recited in claim 3 together with electrical measuringmeans connected between said collecting electrodelead-in and saidenvelope. a source of high potential connected between said high voltagelead-in and said envelope exclusive of said collecting electrode leadin,said envelope being maintained at ground P tential.

5. Apparatus 'for measuring penetrating radiations such as gamma rayscomprising a metallic, vacuum tight sealed envelope, a collectingelectrode and a high potential electrode insulatingly supported thereinby insulators supported by said envelope walls, lead-ins for each ofsaid electrodes, electrical measuring means connected be,- tween saidenvelope and said collecting electrode lead-in, a high potential sourceconnected to said high potential electrode lead-in, said envelope beingmaintained at substantially the same potential as said collectingelectrode and an electrostatic shield inside said envelope for screeningthe interior surface of said collecting electrode insulator from theelectrostatic efiects of said high potential electrode and lead-in.

6. Apparatus recited in claim 5 together with a metallic electrostaticshield exterior of said envelope and between said lead-ins.

7. Apparatus as recited in claim 3' together with a box-like metallicshield exterior of said envelope for surrounding said collectingelectrode lead-in and a second box-like metallic shield integral withsaid envelope walls and surrounding the first-mentioned shield as wellas said high potential lead-in.

8. An ion chamber measuring device comprising, in combination, ametallic vacuum tight casing maintained at ground potential, anionizable gas therein, a collecting electrode,v a glass-tometalsealprotruding exteriorly of said casing for insulatingly supporting saidcollecting electrode from said casing, a high potential electrode, asecond glass-to-metal seal protruding exteriorly of said casing forinsulatingly supporting said high potential electrode from said casing,leadins extending through said glass-to-metal seals, a source of highpotential connected to-said high potential electrode, a cylindricalmetallic guard surrounding said first-mentioned glass-to-metal seal, andan electrical meter interconnected between said collecting electrode andcasing, exteriorly of said casing, for measuring the ion current flow tosaid collecting electrode as the result of ionization of said gas whenthe device is subjected to penetrating radiations such as gamma rays.

BARTON L. WELLER.

REFERENCES Cli'lllElD The following references are of record in the

